st/mesa: fix a regression from cae2bb76

stObj->pt is null when a TFP texture is passed to st_finalize_texture,
and with the changes introduced in the above commit this resulted in a
new texture being created and the existing image being copied into it.

NOTE: This is a candidate for the 7.10 branch.

Reviewed-by: Alex Deucher <alexdeucher@gmail.com>

src/mesa/state_tracker/st_cb_texture.c

@@ -1858,9 +1858,8 @@ st_finalize_texture(struct gl_context *ctx,
     * will match.
     */
    if (firstImage->pt &&
-       stObj->pt &&
        firstImage->pt != stObj->pt &&
-       firstImage->pt->last_level >= stObj->pt->last_level) {
+       (!stObj->pt || firstImage->pt->last_level >= stObj->pt->last_level)) {
       pipe_resource_reference(&stObj->pt, firstImage->pt);
       pipe_sampler_view_reference(&stObj->sampler_view, NULL);
    }